Multifocal lenses should simultaneously meet a number of demands. Initially, a sufficiently good contrast transfer function in two or more focal planes should be ensured. Furthermore, the contrast transfer function should be independent of the size of the pupil. And finally, it should be easy to make the lens; it should not have projections or edges, and therefore have curves which are as smooth as possible.
Such lenses are used, in particular, for correcting visual defects by means of spectacle lenses or as intraocular lenses (IOLs).
In contrast to the monofocal IOLs, which were already introduced many years ago, multifocal lenses were previously only implemented for the bifocal case since there are significant problems in satisfying the aforementioned demands simultaneously. Here, a variant is based upon a special rotationally symmetric ring system, wherein there is sufficiently good imaging for two discrete object-side focal planes, for example at 0 dpt and at a corrective power of approximately 3 dpt, by skillful matching of ring radii, ring widths and ring depths.
Such a bifocal lens is described in U.S. Pat. No. 5,982,543 A and uses a rotationally symmetric Fresnel-like ring system.
U.S. Pat. No. 6,120,148 A describes a rotationally symmetric diffractive ring system. The bifocal lens from U.S. Pat. No. 6,536,899 B1 likewise utilizes a ring system, wherein each ring consists of two sub-rings, which respectively realize the two desired focal lengths.
In a slightly modified form, solutions are also derived herefrom, in which a single lens covers an extended, continuous range of focus. Such lenses are also known by the term “extended depth of focus lens” or else as “EDoF lens”. In United States patent application publication 2006/0176572 A, use is made of a rotationally symmetric system of rings, wherein the individual focal lengths of the rings lie within the desired continuous focal length range. The “extended depth of focus” effect is created by mixing the various focal lengths.
The system in accordance with United States patent application publication 2011/0279912 consists of sectors (“pie slices”) with a focal power increasing in the azimuth direction. Here, the focal power distribution has discrete steps between the sectors.
United States patent application publication 2010/0002310 A1 describes an optical imaging system for a camera which has an extended depth of field range. The extended depth of field is achieved by a combination of several lenses with aspherical surfaces.
A disadvantage in the case of intraocular lenses in particular is that a strong radius curvature is required when using “normal” spherical or aspherical basic lens shapes due to the relatively short focal length caused by the length of the eye. This creates a large lens thickness, a relatively large lens volume with a correspondingly large weight. Since intraocular lenses are manufactured from organic polymers, the refractive index is usually relatively low, leading to a strong radius curvature and hence also a relatively thick lens shape.